Although environmental factors clearly play a role, studies of twins and adoptees show that obesity is a familial trait which to a large degree can be ascribed to genetic factors. According to evolutionary models, obesity-causing variants may originally have had an evolutionary benefit, whereas in a modern environment they pose a risk. Despite a clear genetic cause, the molecular genetic variations underlying common forms of obesity are not clear. Out of many hundred candidates, only a few hold up, and only variations in the MC4R gene have been found with a prevalence which may lead to common forms of obesity.